s4-pydsdb: Use local memory context in py_dsdb_get_oid_from_attid()

s4-pydsdb: Use local memory context in py_dsdb_get_oid_from_attid()

This version reverts changes from commit b974966cc2b4d0b5b0d83206070b5f7c5c6495d1
and is what Matthieu Patou had commited in d784ecec555a3d9737e6f4b3894f27904d2b833c
with added reference to the schema cache.

I think referencing schema here is the right thing to be done
as thus we garantee that schema cache will stay in memory
for the time our function is executed